window: remember image property visibility state

Added GSettings support to Loupe. Inspiration was taken from Fractal: LpApplication will contain gio::Settings and all code needing them will reach them as a singleton by calling g_application_get_default. This design makes the settings dependency less explicit than dependency injection, but allows for shorter code when new settings will be added across different parts of the application.

A show-properties setting is added for remembaring the image property sidebar state. It's read by LpWindow when the current page changes, and saved when toggling the sidebar. When no image is being shown, the sidebar is always hidden, while the setting remains unchanged. The sidebar state will be restored when a picture is opened.

Closes #147 (closed)